1. Field of the Invention
This invention relates to blades for use in turbomachines such as aircraft turboengines, and in particular is concerned with damping vibrations occurring in such blades during engine operation.
2. Description of the Related Art
With large compressor blades, for example fan blades of a by-pass gas turbine engine, it is known to utilise secondary platforms, commonly called snubbers, at locations along the aerofoil portion of the blade to damp vibrations caused by twisting, flutter and flapping of the blade. Snubbers on adjacent blades interact with each other so as to form in effect a continuous platform which damps the vibrations. It will be appreciated that snubbers add undesirable weight to rotating parts, and that the blade roots and the discs or drums on which the blades are mounted have to be considerably strengthened to withstand the resultant high centrifugal forces when the turbine is in use. The manufacturing and machining processes are made more complex by the presence of snubbers, and snubbers furthermore compromise the aerodynamic efficiency of the blades and introduce highly stressed zones at vulnerable regions of the blades.
It is also known to provide turbine blades with tip shrouds which serve both to minimise gas leakages at the blade tips and to provide damping of vibrations of the blades in much the same manner that snubbers do on compressor blades. Here again, turbine rotor assemblies embodying blades with tip shrouds suffer from many of the disadvantages of snubbers outlined above.
It is also known to damp shroudless blades by locating resilient blocks under the blade root platforms, but this requires a larger chord blade to obtain sufficient flexure to enable damping to be effective. This in turn results in increased disc run loads.
Another method of damping a blade employs a closed chamber within the aerofoil portion of the blade and adjacent its tip. The chamber contains a liquid and is shaped, positioned and aligned relative to the blade so that in use the liquid is constrained to form a column which oscillates within the chamber under centrifugal loads at a natural frequency which matches, or is close to, the resonant frequency of the blade to be damped. Such a method is unattractive in view of the current trend towards monocrystalline blades. For example, the provision of a closed chamber containing a liquid within a monocrystalline turbine blade is beset with manufacturing difficulties.